What is the the boost blow off pressure for the 617 turbo diesels? I am getting 7.5-8.0 psi before the waste gate opens. I thought stock was supposed to be 10 psi.

The wastegate opens at 9psi on all stock OM617 of all years. The safety switch closes at 16psi.

editing for clarity, the following is wrong:

"Stock is specified to be 7-9psi and the overboost alda switch engages at either 12psi or 15psi."

Incorrect.

Fun fact: Though specified by Mercedes at 10-11psi stock, ALL engines shipped set at 9psi.

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Interesting. I looked around to see if I could find where I got the numbers, and found that MB mentions 1.7 bar of boost in their SAE paper, which is what I get right now running with no wastegate. I've attached the piece which isn't relevant to the question of factory boost but is interesting anyways. I also have the full SAE paper if anyone wants it, PM me.

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My reading of the excerpt from the SAE article is that they are talking of the wastegate controlled engine, and the 'boost pressure' is the total pressure being exerted on the cylinder walls.
(1.0 bar from the atmosphere and the additional air charge of 0.7 bar for a total of 1.7 bar)

From the graph of boost vs engine speed if you subtract 1.0 bar the engine performance makes sense.
[Turbo lag from 1000 rpm (with 0.2 bar) to 2000 rpm with (0.7 bar), and wastegate controlled boost of 0.7 bar (from 2000 rpm to 4000+ rpm)]
Just a little different interpretation of the definition of boost pressure (in the SAE article) from what we typically define as boost pressure.

Essentially the same graph, in greater detail (with boost expressed in millibars, as the additional amount of pressure over over atmospheric pressure) is attached below.
It is from the W123 FSM, Section 09-450 Functional description of exhaust gas turbocharger with aneroid compensator, on Page 4 of the pdf.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

First, 1.7bar absolute pressure, not gauge pressure.
That is the 0.7bar (psi) specified in the service manual.


Second, running with the wastegate closed is extremely bad. Its there for a reason, that turbo is not designed to operate without a wastegate, you're severely overspeeding the turbo and putting your engine at high risk of damage when it will fail.

Probable, judging from the compressor map I am slightly above or at the maximum speed of the compressor wheel marked on said map. Planning on installing a boost controller later to bring it to heel at 15psi or 17psi before I install a HE221W+intercooler setup.

People have run it up at 17psi after disabling the overboost protection valve. The real issue is the stock compressor wheel acts more like a heater above 13psi and it just isn't efficient. This does increase the chance of the compressor wheel grenading.

10 to 12psi is the sweet spot for the stock turbo. I know people who have run a non stock turbo and intercooled up to 22psi with extra fuel. However that is a deep rabbit hole to dive into.

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white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

With the wastegate disabled, I ran 25psi for a while. Plotting myself on the compressor map, I am quite confident that I was not overspeeding and was just near the top of the map at a pressure ratio of 2.7 and I think mass flow of 20-25lbs/min at ~4000rpm, inside the max compressor RPM limit of ~155krpm. This was quite inefficient and made the engine lose a lot of top end power through the restrictive turbine housing. 12-15psi really does seem to be the sweet spot with a turned up stock pump.



After removing the turbo and looking in the flange, it is evident why it was so restrictive - the turbine housing inlet is very small and the T3 flange is roughly half wastegate and half turbine by area. I will also be rebuilding this turbo and examining the compressor for any sign of orangepeeling or other damages, and might be able to get it under a metallurgy microscope.

What is the advantage of setting the boost to 10.5-11.0 psi? What is the disadvantage?

On a OM617.952 that is the high normal.

At a higher boost pressure, you get more fuel and use more fuel and in theory more fuel gives you more power but the higher boost in a sock set up will only give you a limited amount more power at the high normal end.

Let's suppose you have a stock setup fuel injection pump and managed to get 14 psi of boost. You are not getting any more fuel from the fuel injection pump than you got say at 12 psi so you would not have some surge of power.

To make use of the higher boost you need more fuel. So, some modify their pumps to get more fuel.

Before someone dabbles with that, they ought to have a full understanding of how the whole of it works and their engine and cooling systems need to be in great condition. Adding more fuel and boost increases the pressure in the cylinders, heat generated and stress on parts. And in the long run the engine pays for that.

One of the items that is monitored is the exhaust gas temperature.

However, I am not the one to ask about souping up the engines. There is a forum called the superturbodiesel forum where the more extensive knowledge is.

This is a slightly different area. Some people want better acceleration/power at lower speeds. No expert at that either but the general trick is to get the boost pressure to rise at lower engine rpms and the higher boost pressure causes more fuel to be injected at that lower rpm. I have a general idea that people have used different sized compressor wheels and perhaps exhaust turbines to do that, and they have something called variable venturi turbo chargers. They restrict the exhaust gas flow, and the higher pressure of the exhaust gasses causes the turbo to turn faster sooner. Later they open up and have a normal gas flow.

Anyway, there is a lot to learn.

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84 300D, 82 Volvo 244Gl Diesel